The present invention, which is a result of a contract with the U.S. Department of Energy, relates generally to the art of microwave heating control systems and more specifically to the control of boiling of a liquid in a microwave applicator.
It is frequently useful to boil down liquids and chemical slurries in open containers inside microwave ovens or other microwave applicators in order to study the behavior of microwaves on the boildown process and to measure the off gases produced by this process. The boildown process is best done in a controlled fashion whereby the microwave power absorbed by the liquid is just equal to the heat required to vaporize the liquid at 1 atmosphere pressure. However, continued microwave heating at full power results in uncontrolled boiling and subsequent splattering of the liquid out of the container. For example, lowering the microwave power in a microwave oven offers some reduction in splattering but never completely eliminates the problem because lower power levels are achieved with full power switched on and off at fixed intervals to give the desired average power. The shortest "on" time available in most commercial ovens is about 1 second, and at the boiling threshold of the liquid even a 1 second full power pulse is enough to cause some splattering. Moreover, even a microwave generator and applicator with continuous power control must be constantly monitored by the operator to control the level of boiling instabilities in the liquid. Thus it will be seen that there is a need for a means of controlling the level of boiling of a liquid in a microwave oven or in a microwave applicator.